(1) Field of the Invention
The present invention relates to the synthesis of sound and particularly to the excercise of control over the envelope of an alternating current signal which is to be transduced into sound in an electronic musical instrument. More specifically, this invention is directed to digital logic circuitry for controlling the envelope of a signal produced in a polyphonic musical synthsizer. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Brief Description of the Prior Art
Electronic musical instruments wherein the sound to be created is synthesized from digitally coded information are known in the art. Such digitally operated musical instruments are disclosed, by way of example, in my co-pending applications Ser. Nos. 158,251 and 235,843. The synthesizers disclosed in the referenced co-pending applications produce audio frequency signals through the sampling of the content of "phase counters" and the subsequent integration of the outputs of such "phase counters". Employing the synthesizers of the co-pending applications, the polyphonic generation of audio frequencies is possible and, for purposes of discussion, it may be assumed that eight sounds are to be reproduced simultaneously. As the term "sound" is employed herein it means an individual basic frequency plus the harmonics of that basic frequency which may be characteristic to the particular instrument to be simulated. There may, depending on the instrument, be eight or even ten harmonics and the individual frequencies of these harmonics will hereinafter be referred to as "individual sounds". Thus, a "sound" having five harmonics will comprise six "individual sounds".
In the electronic production of musical sound the harmonic content of the "sound" is but one of several criteria which must be met in order to produce the requisite audible information. Thus, it is equally important that the appropriate envelope curve be generated, i.e., the amplitude variation over time including both the "attack" and "decay" portions of the curve. In other words, the shape of the envelope will vary for the same basic frequency from instrument to instrument and these variations must be taken into account in the synthesis process. Also, in addition to the amplitude variations, there are also frequency variations, i.e., the characteristic chord instrument vibrato.
To summarize the preceding brief discussion, a musical synthesizer must be able to generate two hundred or more different envelope curves in order to take into account the possible sounds the user of the electronic instrument may desire to produce. The typical prior art musical synthesizer generates an envelope curve which takes into account the attack and decay phases of just one individual sound while the remaining individual sounds which are being simultaneously reproduced remain unaffected in amplitude and frequency. If a plurality of envelope curves must be simultaneously produced, accordingly, the number or circuits employed for this purpose must be multiplied by the number of curves to be simultaneously produced.
A proposed synthesizer circuit is disclosed in U.S. Pat. No. 4,083,285. The circuit of this patent would permit variation of the envelope curves of the harmonics of a "sound" in addition to the basic "individual sound". This patent also suggests the use of a "sound color memory" which holds data during the "attack" period when the envelope is varying from initiation of the "sound" in the customary or predetermined manner, during the decay period, and also holds maximum amplitude and sustain amplitude data. The data in this "sound color memory" is sampled in a time multiplex mode and applied as an input to a control unit. While the use of a "sound color memory" reduces the memory capacity required in the apparatus, the above-discussed problem of prior art synthesizers is not overcome since the ability to vary envelope curve shape is quite restricted. This restricted ability to vary envelope shape results from the fact that only a single envelope, although one which is frequently desired, may be generated. For further amplitude shapes, including amplitude modulation, frequency modulation, repetition, etc., additional complex circuits would be required.